Apparatus for the manufacture of concrete products

ABSTRACT

An apparatus is provided for the manufacture of concrete products in which a mixture comprised of cement, aggregate, a vinyl acetate-dibutyl maleate copolyer and an amount of water sufficient to make a relatively dry mixture is initially compressed into the desired configuration. This compressed-shaped article is then cured in a chamber charged with carbon dioxide gas to provide a fully cured concrete product having excellent physical properties. The apparatus of this invention is especially useful in the manufacture of terrazo panels and the like.

This is a continuation of application Ser. No. 817,263, filed July 20,1977 now abandoned which in turn is a continuation of application Ser.No. 468,566 filed May 9, 1974, now abandoned, which in turn is acontinuation of Ser. No. 317,060 filed Dec. 20, 1972, now abandoned,which in turn is a divisional application of application Ser. No.106,364, filed Jan. 14, 1971 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for the manufacture of concreteproducts. More particularly, this invention is concerned with themanufacture of terrazzo panels and the like.

2. Description of the Prior Art

The term Concrete as it is employed in this specification refers to thestone-like product obtained by blending a cement, aggregates such assand, gravel, crushed stone and the like, and water together inappropriate amounts, and allowing the mixture to harden and cure. Theterm Terrazzo as used in this specification refers to a special form ofconcrete in which the aggregate used is selected so as to impart adecorative effect to the final product when exposed by grinding andpolishing. The aggregate used in terrazzo can include, for example,marble dust, marble chips, granite chips, colored stones of varioustypes, broken glass and the like.

In the prior art methods of manufacturing concrete products, the cement,aggregate, additives if any, and water are blended in various ratios. Ingeneral, a sufficient amount of water is added to completely hydrate thecement and also impart a degree of fluidity to the blended mixture. Theblended mixture is then poured or compacted into a form and allowed totake an initial set. The initial set usually occurs within one-half hourto four hours, depending upon the temperature and the particularconcrete mixture employed. The concrete product, after this initial set,is solid, but has very little strength. In order to obtain the maximumstrength possible with a given concrete composition using the prior artmanufacturing techniques, the concrete is generally allowed to cure fora period of time from several days to several weeks.

The excessively long period of time required for both the initial set,and particularly for the development of the ultimate strength, causesconsiderable problems in the manufacture of cast concrete products. Theconcrete mixture must remain in the mold until the product acquiressufficient strength that it will not fall apart when the mold isremoved. Furthermore, the concrete product, even after the mold isremoved, must be allowed to cure for an extended period of time beforeit is subjected to any substantial stresses. The need to maintain theconcrete products in the mold until the initial set is obtained, and forcuring the products until the ultimate strength is developed requires asubstantial inventory of molds and a rather large storage area for theproducts being processed. Of even greater consequence is the costinvolved in the prior art processes since the handling of the weakproducts obtained after the initial set and the storage of the productfor several days or weeks until fully cured is quite costly in bothmanpower and storage space.

The problems encountered in handling and storage of concrete productsusing prior art processes can best be illustrated by the conventionalmethods of manufacturing terrazzo panels. In the conventional method aconcrete mixture containing the desired type of aggregate is eitherpoured or compacted into a suitable panel mold. The mold is then storeduntil the mixture initially sets. At this point the panel cannot beground or polished since the machining would cause the panel todisintegrate, especially if the grinding were conducted at the highspeeds required for commercial production of terrazzo panels.Accordingly, the panel has to be stored for days or weeks until fullycured, and then finished.

A further problem encountered with cast concrete products in general,and terrzzo products in particular, is that often, using the prior artprocesses, products are obtained which lack sufficient compressivestrength. In order to compensate for the relatively low compressivestrength, the products have to be cast with thicker cross sections so asto provide the required strength. This materially adds to the cost ofthe product, the cost of transportation of the finished product, and thecost of installing the product. As can be seen most readily withterrazzo panels which are used as floorings in many public buildings andthe like, if the panels can be made stronger, thinner sections can beused as these would have the same compressive strength as the weaker,thicker panels. The use of the thinner terrazzo floor panels directlyreduces the material cost of the flooring, and indirectly would reducethe total cost of the building, in that the supporting structures forthe flooring would not have to be as strong in order to support thelighter weight terrazzo panels.

Attempts have heretofore been made to decrease the cure time in themanufacture of concrete products. These included adding various types ofaccelerators to the concrete mixtures as well as heating or steaming theconcrete products during manufacture. These methods at best producedmarginal improvements with regard to reducing the cure time, and weresomewhat expensive. For example, in high pressure steam curing theproduct often had to be steamed for twelve to twenty-four hours in orderto obtain a satisfactory hardness and strength in the final product. Inaddition, certain of the processes had adverse effects on thecomposition. High pressure steaming of the concrete products containingsubstantial amounts of marble or limestone, for example, caused a verymarked decrease in the ultimate compressive strength of the finalproduct. Accordingly, these processes could not be used with terrazzoproducts which usually contain substantial amounts of marble dust ormarble chips.

In order to increase the strength of concrete products certain additiveshave heretofore been suggested. These additives include materials whichto some extent plasticize the finished concrete product so as toincrease its compressive strength. Additives have been suggested toimprove the properties of concrete products, including bituminousmaterials and various types of rubbers and emulsions of syntheticresins. One patent which has been found to be particularly interestingbecause it is concerned primarily with terrazzo-type concrete productsis Murray, U.S. Pat. No. 3,305,506. The Murray patent teaches the use ofa vinyl acetate-dibutyl maleate copolymer for improving the compressivestrength. It should be noted, however, that while these additives dohave a definite beneficial effect on improving the compressive strength,the ultimate compressive strength is still relatively low. This limitsthe use of concrete products having thin cross sections in applicationswhich require the material employed to carry considerable compressiveloads.

It is accordingly an object of this invention to overcome theaforementioned problems and difficulties of the prior art.

It is the more specific object of this invention to provide an apparatusfor rapidly curing concrete products.

It is an additional object of this invention to provide an apparatus forproducing concrete products with improved strength.

It is a still more specific object of this invention to provide anapparatus for continuously manufacturing terrazzo-type concreteproducts.

Other objects and advantages of this invention will become furtherapparent from a study of the attached figures and a continued reading ofthe specification and subjoined claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration in cross section of an apparatus ofthis invention which is especially useful for the manufacture ofterrazzo panels.

FIG. 2 is an enlarged view of the dotted area 2 of FIG. 1.

FIG. 3 is a graphic illustration of the temperature rise and timerelationship in the method of this invention.

SUMMARY OF THE INVENTION

The objects of this invention have been achieved by providing anapparatus wherein cement, the desired aggregate, a vinyl acetate-dibutylmaleate copolymer and a minor amount of water are initially blended.This relatively dry mixture is then compressed into the desired shape ina mold. The shaped article is then fed through a chamber charged withcarbon dioxide gas which causes a rapid exothermic reaction to occurduring which the article is fully cured in a matter of a few minutes.The cured product exhibits substantially improved physical properties,and in particular, compressive strength as compared to similar productsproduced in the conventional manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention the starting mixture is comprised of fourprincipal ingredients, namely, portland cement, aggregate, a copolymerof vinyl-acetate and dibutyl maleate, and water.

The cement which is used can be the common commercially available types.In this specification specific reference will be made to portland cementbut it should be appreciated that other similar hydraulic cements canlikewise be used in the method of this invention. Portland cementcompositions are comprised of about 60-70% by weight of CaO, 17-25% byweight of SiO₂, 3-8% by weight of Al₂ O₃, minor amounts of other oxidessuch as Fe₂ O₃, MgO, Na₂ O, K₂ O and about 1-3% by weight of SO₃. As inall concrete-forming processes, it is important that the cement be keptas dry as possible prior to the formation of the starting mixture sincethe oxides comprising the cement will hydrate in the presence of water,and a weaker product will be obtained if the cement is partiallyhydrated prior to formation of the shaped article from the mixture.

The aggregate can be selected from an almost unlimited varietyofdifferent materials. The most commonly employed aggregates for use inthis invention are sand, gravel, marble dust, marble chips, granitedust, granite chips, and crushed limestone. Additional other types ofmaterials which can be advantageously employed as aggregates are sealshells, broken glass, especially colored glasses, and masonry-typebuilding rubble such as crushed concrete, mortar and crushed bricks. Ascan be seen from the above, the choice of the particular aggregate whichcan be used in the method of this invention is quite extensive.

The selection of the particular aggregate is dependent on a combinationof several factors. These include the physical properties desired in thefinal concrete product; the decorative effect desired in the finalproduct; and the availability and cost of the aggregate. The particularaggregate or combination of aggregate employed has a marked effect onthe ultimate strength of the concrete product. As is well known, neatcement when mixed with water and allowed to set and cure will develop acertain minimal amount of strength. However, when an aggregate is added,the aggregate reinforces the product and results in a product having aconsiderably higher ultimate strength. The strength of concrete productsis dependent to a large extent upon the strength of the aggregateemployed, with stronger aggregates resulting in stronger concreteproducts.

The particle size of the aggregate, or blend of aggregates employed, islikewise quite important. The use of large-size aggregate particlestends to increase the strength of concrete products. However, the use ofonly a large particle size aggregate can result in a relatively weakproduct since a considerable volume of air voids will form between theaggregate particles and considerably weaken the product. Accordingly,the use of a combination of relatively fine aggregates and coarseaggregates in the proper balance will significantly enhance the ultimateproperties of the final product.

The appearance desired in the final product is another factor toconsider in the selection of the aggregate, especially if a terrazzoproduct is to be produced, since the aggregate will be exposed as aresult of finishing. In producing terrazzo, marble dust is most commonlyused as the fine aggregate, and marble chips as the coarse aggregate.The color of the marble is varied according to the effect desired in thefinal product. Terrazzo products, however, are not limited to the use ofmarble as the sole aggregate. Quite striking effects are obtained withaggregates such as sea shells and broken glass which when ground andpolished add distinctive effect to the terrazzo panels.

While this invention is especially well suited to the preparation of therelatively exotic concrete products such as terrazzo panels, it shouldbe appreciated that this invention can also be used to make buildingblocks and the like. Because of the relative speed and simplicity of themethod of this invention, as will be explained in greater detail below,common masonry rubble which is obtained on tearing down an existingstructure can be crushed and used as an aggregate to form buildingblocks for constructing new buildings.

One of the more essential components of the starting mixture of thisinvention is a copolymer of vinyl-acetate and dibutyl maleate. Thisparticular copolymer appears to have a dual effect in this invention.Initially it makes possible rapid cure of the starting compositions toobtain the final fully cured product. When the copolymer is not includedto the mixture, rapid curing is not obtained. Secondly, as disclosed inMurray, U.S. Pat. No. 3,305,506, the copolymer has a definite beneficialeffect on the strength of the product. The relative proportion of themonomers comprising the copolymer can be varied considerably and stillobtain satisfactory results. The optimum results, however, have beenobtained with copolymers which are comprised of about three parts byweight of the vinyl-acetate monomer and one part by weight of thedibutyl maleate monomer. A commercially available copolymer of this typeis available which is comprised of about 77% by weight of vinyl acetateand about 23% by weight of dibutyl meleate prepared in emulsion formusing polyvinyl alcohol as a protective colloid.

The final essential starting material for the mixtures employed in thisinvention is water. Water, as is well known to those skilled in the art,is required for hydration of the oxides comprising the portland cement.In the prior art processes, an amount of water had to be used which wasat least theoretically required for complete hydration of the oxidescomprising the portland cement. In general, a considerable excess wasemployed in order to compensate for the amount of water required forwetting out of the aggregate and also to provide a somewhat fluidmixture to facilitate the handling of the concrete mixture. As will bepointed out below in greater detail, in this invention a considerablylesser amount than the theoretical amount of water required for completehydration is employed and, quite surprisingly, a superior product isobtained. The water used in this invention does not require anyparticular pretreatment providing it is of the ordinary qualitygenerally employed to make concrete.

As was pointed out above, the proportions of the ingredients comprisingthe starting composition are quite important. The starting mixtureshould generally contain from about 60-75% by weight of the aggregatewhich can be a combination of both fine aggregate and coarse aggregatein proportions required for obtaining the desired strength anddecorative effect in the final product. The size of the aggregate isselected so that the starting mixture can readily be compressed into agiven shaped article, relatively free of voids, with preferably lessthan 1-4% by volume of voids being present in the compressed product.The mixture further contains from 20-25% by weight of cement.

The two remaining essential components, that is, the copolymer and thewater, are present in relatively minor amounts. The vinyl-acetatedibutyl maleate copolymer is present in an amount of from about 1-2%based on the solids contents of the resin.

The total weight of the water in the composition is limited to about5-10%, and more preferably 4-8%, with optimum results being obtainedwith about 6% water. In determining the amount of water to be added tothe mixture, the water, if any, associated with the aggregate, as forexample when wet sand is used, and the water present in the polymeremulsions, must be taken into consideration. The mixture may alsocontain other additives such as dyes, pigments and the like.

The preferred method of preparing the starting mixture is to initiallypremix the dry components, that is, the cement and the aggregate, sothat a uniform blend is obtained prior to the addition of the liquidingredients. Then, because of the relatively minor amount of the liquidsused, it is preferable to spray the dry mixture with the mixture of thecopolymer and water. The mixing is then continued until the totalmixture is uniformly blended. The mixture at this point will berelatively dry appearing as compared to the fluid, soupy mixturesgenerally employed to make concrete poducts. The mixture should have therelatively stiff consistency similar to drained wet sand or the like.

The particular type of blending apparatus employed to obtain thestarting mixture is not critical providing it sufficiently blends thestarting materials. One type of blender which has, however, proven to besatisfactory is a double screw blender since this provides a verycomplete mixture of the starting materials in a short period of time. Inaddition to the blender 10, a screw feed can be provided in thefeed-line 12 to both further mix the ingredients of the mixture and alsoto facilitate the transfer and metering of the mixed material to thepress 14.

The press 14 is one of the more essential parts of the apparatus of thisinvention. It is a high pressure hydraulic press which is capable ofexerting over a million pounds of pressure per square yard on thematerial being compressed. Positioned below the ram 16 of the press 14is a cavity mold 18. The cavity mold shown is for making flat panels.The cavity mold has high walls in order to receive a sufficient amountof the starting mixture to form a panel of the desired thickness aftercompression. Associated with the cavity mold 18 is an ejection means 20for removing the pressed, uncured panel 22 from the cavity mold 18.

The mixed starting material is fed into the cavity mold 16 in a weighedamount sufficient to make a panel of the desired or predeterminedthickness or height thickness. The cavity mold 18 is positioned belowthe ram 16, and the press is activated. The ram 16 compresses themixture into a compacted panel 22 which has sufficient structuralintegrity to be removed from the cavity mold by the ejector means 20without breaking apart. The uncured panel 22 at this point isfree-standing and capable of being handles, but it should be appreciatedthat if dropped or if roughly handled it can be readily broken. Theability to form an unset, uncured panel 22 which does not require theuse of a form to maintain its shape during further processing isbelieved to be due to the combination of the use of the high pressure inthe molding process and the presence of the copolymer in the mixture aswell as the use of a relatively low amount of water in the mixture.Because the starting material can be formed into the desired shape, andthe mold thereafter immediately removed from the shaped article, it isnot necessary to maintain a large inventory of molds as was required inthe prior art processes.

The uncured panel 22 is advanced into a carbon dioxide chamber 24 (FIG.2) which is clearly the most important single part of the apparatus ofthis invention. The chamber 24 should be built so as to be relativelygas tight. Furthermore there should be included flexible seal means 26,28 at the entrance and exits of the chambers so as to minimize thecarbon dioxide losses as the panels 22 enter and leave the chamber 24.As illustrated, the panels 22 are supported by a conveyor belt 30 whichis of an open mesh construction in order to permit carbon dioxide gas topass through the belt and contact the bottom portions of the panels 22being processed through the chamber 24.

Inlets 32 for carbon dioxide gas are provided at both the top and bottomsides of the chamber 24 opposite each other. As illustrated, only threeinlets 32 are shown on each side of the chamber. However, it should beappreciated that a considerably larger number of inlets 32 arepreferably employed to introduce the carbon dioxide gas into the chamber24 As shown in FIG. 2, the conveyor belt 30 is disposed in the carbondioxide chamber spaced above the bottom of the chamber and the bottomside inlet 32 is disposed below the level of the conveyor belt 30.

The relative height of the chamber 24 is a highly critical factor. Inorder to obtain the maximum benefits of this invention, the chamber 24should not be excessively large. Preferably the height of the chambershould be at most from 11/2-4 times the thickness of the panel beingprocessed. The rate and the depth of the cure is directly dependent uponthe chamber height. This was found in test wherein panels were curedunder substantially the same conditions with the exception of the heightof the curing chamber. It was found that a 1° thick panel would rapidlycure completely throughout its entire volume in a 4° chamber in about2-5 minutes. It was further found that the same type of panel when fedthrough a chamber having a considerably greater height, namely, about18°, would at best skin cure and would have a soft, uncured centerportion. Further, as a result of this finding, the height of the chamberwas gradually reduced and it was found that the proportion of theuncured center section was directly dependent upon the height of thechamber. As the chamber height was reduced, more of the panel cureduntil the height of the chamber was reduced to a height wherein theentire thickness of the panel cured. It was further found that evenextending the time of cure in chambers having an excessive height had nosubstantial effect on the degree of cure obtained. The reason for thecriticality of the chamber height is no known.

The carbon dioxide can be obtained from any convenient source. Bottledcarbon dioxide gas is the most conveniently used in most installations.The gas should be comprised essentially of carbon dioxide with at mostminor amount of other gases being present in the carbon dioxide. Thepressure of the carbon dioxide which is maintained inside the chamber isnot particularly critical, with approximately the same results beingobtained with pressures from 1-50 PSIG pressure in the chamber. What isimportant, however, is that the carbon dioxide be maintained at asufficient positive pressure that it will prevent the infusion ofatmospheric air into the chamber. The composition of the atmosphereinside the chamber 24 should be essentially carbon dioxide. Under idealconditions the carbon dioxide content of the atmosphere inside thechamber 24 should be at least 95%, and preferably higher, for example98% plus.

In the invention, as the panel 22 is fed through the chamber 24 chargedwith carbon dioxide, there is a sudden exothermic reaction wherein thetemperature rises about 20°-60° F. depending upon the mixture employed.The exothermic rise occurs in about 11/2-5 minutes. The rate oftemperature rise then markedly levels off and rises gradually to amaximum temperature which is generally 5°-10° higher above the levelingoff point.

What is highly surprising, as is shown in FIG. 3, is that unless a rapidexothermic temperature rise is obtained with a leveling off point beingreached in less than about 5 minutes, a poor, partially cured product isobtained. For example, when using an oversize chamber the temperaturerise will be substantially linear as shown by the dotted line on FIG. 3.Approximately the same maximum temperature will be reached in about 10minutes. However, unlike the sample, which rapidly exotherms, as shownby the solid line on the graph, the samples represented by the dottedline will be poorly cured or simply skin-cured, with a soft, uncuredcenter portion.

It should be noted that the carbon dioxide is an essential reaction inthe process of this invention. The rapid exothermic reaction with theresulting rapid cure is not obtained with other gases such as air,oxygen, nitrogen, etc. In addition, it should be further noted that heatalone is not sufficient to cause the rapid cure. When panels of thecomposition disclosed above are heated by external means to the sametemperature as that reached in the carbon dioxide chamber, there is nosubstantial curing of the panel.

It should be also appreciated that there is a close interrelationshipbetween the amount of water used in the starting mixture, the copolymeremployed, and the carbon dioxide. If an amount of water substantially inexcess of that defined above is employed, the mixture will not cure inthe carbon dioxide chamber. This is believed to be due to a lack ofrelative solubility of the carbon dioxide in water, and also because theexcess water may prevent the carbon dioxide from penetrating into theconcrete mixture being cured.

What is even more surprising, as noted above, is that if thevinyl-acetate diutyl maleate copolymer is not included in the mixture,rapid curing will not be obtained. The exact reason the copolymer has tobe in the mixture is not known.

A further surprising effect of this invention is that in spite of thefact that a relatively low amount of water is employed in the mixture,often after curing has occurred the cured material will have free wateron its surface. This is believed to be due to a combination ofinterrelated reactions. The initial reaction is believed to be thehydration of the oxides of the cement by the water, which is thenfollowed by a rapid carbonization of the resulting hydroxide by thecarbon dioxide. The result is that the hydrate formed on hydration ofthe oxide is then converted into the corresponding carbonate, and thewater initially used in the hydration reaction is freed and can reactwith additional unreacted oxide. In theory, if both the hydrationreaction and the carbonization reaction go to completion, the entireamount of water added to the starting mixture should be recovered asfree water. This would, of course, account for the presence of water onthe surface of some cured materials. While the above theory appears tobe the logical explanation of the chemistry involved, it should beappreciated that applicant does not intend to limit his invention to theabove theoretical explanation of the process.

The products obtained in accordance with the method of this inventionare quite unique with regard to their physical properties. Whenidentical samples of various concrete compositions were pressed as notedabove, and one sample of each composition was cured according to themethod of this invention, and the other sample was cured for 28 days inthe conventional manner, the samples prepared according to the method ofthis invention had 2-4 times greater compressive strength. This increasein compressive strength is quite striking and surprising.

The rapid curing with the resulting improved properties has made itpossible to continuously manufacture concrete products which are readyfor immediate use as soon as they are cured in the carbon dioxidechamber. The advantages of this invention are especially important inthe manufacture of terrazzo products. As shown in FIG. 1, the panel 22immediately after curing can be finished by grinding and polishing. Thecompleted terrazzo panels are made in a matter of minutes, not days orweeks as in the prior art processes. In addition, the panels aresuperior in all properties to the prior art panels.

This invention has been described wit particular emphasis on themanufacture of terrazo panel products. However, the use of the apparatusof this invention clearly is not limited to the manufacture of terrazzopanels. It can be used to produce a wide variety of different types ofconcrete products. One use of the apparatus of this invention which isespecially promising is on-site manufacturing of building blocks. Theapparatus employed in this invention can be made sufficiently mobilethat it can be set up in a central location in an area which is to beredeveloped. When old buildings are now torn down, the masonry rubble isgenerally hauled away at considerable cost and simply dumped as fill inremote areas. However, using the apparatus of this invention, themasonry rubble can be crushed and then used as aggregate in theabove-described mixture. The mixture of the masonry rubble, cement,copolymer and water is then pressed into building blocks and cured, asdescribed above, in a carbon dioxide chamber. The blocks are then readyfor immediate use in constructing new building in the redeveloped area.Using the apparatus of this invention, the cost of the buildingmaterials is substantially reduced, and the recycling of the buildingrubble also eliminates the problem of hauling away the rubble anddisposing it.

The following examples are given by way of further illustration of thepresent invention are not intended in any way to limit the scope of thepresent invention beyond that of the subjoined claims. All parts andpercentages expressed in the following examples are parts andpercentages by weight, not volume, unless otherwise indicated.

EXAMPLE 1

A mixture for manufacturing terrazzo panels was prepared by blendingtogether 21.0 kgs. of #0 cedar green marble chips, 24.5 kgs. of #1 blueand green marble chips, 67.2 kgs. of #75 white marble dust & 32.8 kgs.of portland cement.

The mixture was blended until uniform, and then an aqueous mixturecomprised of 3 parts by weight of water and 1 part by weight of thecopolymer consisting of 77% by weight of vinyl acetate and 23% by weightof dibutyl maleate was sprayed onto the mixture until the mixturecontained 6% water and 2% copolymer, based on a total weight of themixture.

The mixture was compressed into panels approximately 36"×36" and 1"thick. The panels were virtually free of air voids. The panel was thenejected from the mold and found to have sufficient structural integritythat it retained its pressed shaped after the supporting structure ofthe mold was removed. However, the panel could readily be reduced to theuncompacted state by simply crushing it with a hammer or the like.

The compressed shaped panel was supported on a chain mesh conveyor beltand fed through a gas chamber having a height of 4". This chamber wascharged with carbon dioxide gas introduced into the chamber through aseries of orifices positioned approximately 6" apart from each otheralong both the top and the bottom of the chamber. The pressure of thecarbon dioxide in the chamber was maintained at 10 PSIG. The speed ofthe conveyor belt was set so that the panels to be cured remained in thechamber for 10 minutes.

Heat probes were positioned on the surface of the panels and in theinterior of the panels, and the temperature was monitored. The rise intemperature was recorded as follows:

                  CHART 1                                                         ______________________________________                                        Interior Temperature ° F.                                                                   Surface Temperature ° F.                          ______________________________________                                        0 minute                                                                              74               74                                                   1 minute                                                                              74               74                                                   2 minutes                                                                             78               84                                                   3 minutes                                                                             128              90                                                   4 minutes                                                                             134              97                                                   5 minutes                                                                             136              109                                                  6 minutes                                                                             139              112                                                  7 minutes                                                                             142              113                                                  8 minutes                                                                             145              114                                                  9 minutes                                                                             147              114                                                  10 minutes                                                                            147              115                                                  ______________________________________                                    

The reason for the substantial difference between the interiortemperature and the exterior temperature was believed to be caused by aninitial surface cooling of the panel by the freshly introduced carbondioxide gas. It should be noted, however, that in both temperaturemeasurements there was a sudden exothermic increase in the temperaturein about 5 minutes. The temperature leveled off after this initialexotherm, and came to a substantial equilibrium. In similar tests, whenthe temperatures were monitored for longer periods of time, it was foundthat the maximum temperature is reached in about 10 minutes. whereuponthe temperature gradually starts to decline. This is believed to showthat the reaction is substantially completed within 10 minutes or less.

The panels were evaluated for cure by initially testing the hardness.Thereafter the panels were cut in half to determine if the panels werefully cured throughout their entire thickness. The panels cured as abovewere both very hard and also fully cured throughout their entirethickness. The panels immediately after curing were ground with highspeed diamond wheels, using water as the cooling fluid, and thenpolished. The samples ground very well. The surface of the panels wasexamined and it was found that there was no erosion of the cement frombetween the aggregate particles as is encountered with poorly curedpanels.

EXAMPLE 2

The terrazzo mixture described above was compacted and the panel wasthen stored for 4 weeks at 70° F. This sample was then evaluated and wasfound to be relatively weak. On grinding, the cement eroded from thesamples, indicating that the cement was at best only partially hydrated.The above procedure was repeated with the exception that the amount ofwater was doubled from 6 to 12%. The sample, after 4 weeks of aging, wasconsiderably stronger than the sample produced with 6% water. However,it still had only about 1/3 the compressive strength of the sampleproduced in Example 1.

EXAMPLE 3

Example 1 was repeated, with the exception that the vinyl-acetatedibutyl maleate copolymer was omitted from the mixture. The panel whenmonitored during the carbon dioxide treatment did not show any exotherm.The sample after 10 minutes in the carbon dioxide chamber was removedand evaluated. The sample had not cured and could readily be broken.

EXAMPLE 4

A panel of the composition described in Example 1 was heated between apair of electrically heated plates until the interior temperaturereached 147° F. which is the maximum temperature reached in Example 1.This sample was then evaluated and it was found that it was not cured,which indicated that temperature alone is not involved in the curingprocess of the present invention.

EXAMPLE 5

The procedure for Example 1 was followed, except that the height of thecarbon dioxide chamber was adjusted to 18". The temperature was againmonitored. The surface temperature followed an approximately linearrise; the interior temperature only rose slightly. When evaluated, thesamples were found to be skin-cured about 1/8" deep from both sides, andthe interior of the sample was soft and completely uncured.

The height of the chamber was gradually reduced 2" at a time, andsamples were evaluated at each height. As the height was reduced, theamount of the uncured center portion likewise was reduced, until atabout 4° a fully cured panel was obtained.

EXAMPLE 6

The procedure of Example 1 was repeated, except that the followingmixture was used: 38 kgs. quartz crystal dust, 92 kgs. 50 mixed cedargreen and light green marble chips, 38 kgs. marble dust & 70 kgs.portland cement. The temperature was monitored as in Example 1 and wasfound to be as follows:

                  CHART 2                                                         ______________________________________                                        Interior Temperature °F.                                                                    Surface Temperature ° F.                          ______________________________________                                        0 minutes                                                                             69               74                                                   1 minute                                                                              74               74                                                   2 minutes                                                                             74               82                                                   3 minutes                                                                             123              89                                                   4 minutes                                                                             131              97                                                   5 minutes                                                                             136              103                                                  6 minutes                                                                             139              108                                                  7 minutes                                                                             142              110                                                  8 minutes                                                                             144              111                                                  9 minutes                                                                             146              112                                                  10 minutes                                                                            147              112                                                  ______________________________________                                    

The panels were fully cured and very hard. Some difficulty wasencountered in grinding because of the exceptional hardness of thequartz particles.

EXAMPLE 7

The procedure of Example 1 was repeated except that the followingmixture was employed: 23.6 kgs. #0 white marble chips, 21 kgs. groundlimestone, 21 kgs. #75 marble dust. 25.2 kgs. portland cement & 7 kgs.of a 3:1 mixture of water, and the copolymer of Example 1. The panelswere cured in the manner described in Example 1. The temperaturereadings at the interior of the samples were taken and were found to beas follows:

                  CHART 3                                                         ______________________________________                                        Time               Temperature ° F.                                    ______________________________________                                        start              67                                                         1/2 minutes        68                                                         1 minutes          74                                                         11/2 minutes       79                                                         2 minutes          86                                                         21/2 minutes       93                                                         3 minutes          103                                                        31/2 minutes       110                                                        4 minutes          115                                                        41/2 minutes       120                                                        5 minutes          125                                                        51/2 minutes       130                                                        6 minutes          134                                                        61/2 minutes       138                                                        7 minutes          140                                                        71/2 minutes       143                                                        8 minutes          146                                                        81/2 minutes       147                                                        9 minutes          149                                                        91/2 minutes       150                                                        10 minutes         152                                                        ______________________________________                                    

The samples were fully cured and very hard. The panel ground easily, andthe surface was exceptionally smooth.

EXAMPLE 8

Example 7 was repeated except that in place of maintaining the carbondioxide pressure at 10 PSIG, the pressure of the carbon dioxide withinthe chamber was raised to 60 PSIG. The interior temperature during curewere recorded and found to be as follows:

                  CHART 4                                                         ______________________________________                                        Time               Temperature ° F.                                    ______________________________________                                        0 minutes          68                                                         1/2 minutes        82                                                         1 minutes          93                                                         11/2 minutes       102                                                        2 minutes          120                                                        21/2 minutes       134                                                        3 minutes          142                                                        31/2 minutes       146                                                        4 minutes          149                                                        41/2 minutes       151                                                        5 minutes          152                                                        51/2 minutes       153                                                        6 minutes          153                                                        61/2 minutes       153                                                        7 minutes          153                                                        71/2 minutes       153                                                        8 minutes          153                                                        81/2 minutes       153                                                        9 minutes          153                                                        91/2 minutes       153                                                        10 minutes         154                                                        ______________________________________                                    

The panel after curing was found to be completely cured and hard. It wasequivalent to the panel produced in Example 7.

EXAMPLE 9

Building blocks were prepared from the following mixture: 20 kgs.crushed concrete, 22 kgs. crushed burnt face brick, 25 kgs. groundlimestone, 25 kgs. portland cement, 2 kgs. vinyl acetate-dibutyl maleatecopolymer, 6 kgs. water. The mixture was prepared as in Example 1 andpressed into blocks 8"×12"×21/2" thick. The blocks were cured at 10 PSIGin a 4"-high carbon dioxide chamber for 10 minutes. The blocks werefound to be fully cured and could be used immediately for the formationof walls or the like in a building.

I claim:
 1. In an apparatus for manufacturing terrazzo concrete productsfrom a moist mixture containing copolymeric additive and which mixtureis compressed to form uncured free-standing shaped members ofpredetermined height and then cured in a dry atmosphere essentiallyconsisting of carbon dioxide, a carbon dioxide chamber constructed to berelatively gas tight and having an entrance and an exit provided withflexible seal means adapted to allow passage of said shaped memberswhile maintaining the carbon dioxide content of the atmosphere withinsaid chamber to about at least 95%, the improvement which comprises theheight of said chamber being in the range of 11/2 to 4 times thepredetermined height of said shaped members.